ABSTRACT
As-reduced rare-earth metals, whether produced metallothermically or by fused-salt electrowinning, are generally 98-99% pure. It is usually possible in certain cases, if care is taken to use pure raw materials and adopt controlled environment processing, to restrict the total impurities in the as-reduced metals to about 0.5%. More often than not, this additional gain in purity either is not technically possible or involves a great increase in process cost, especially in commercial-scale operations. The general alternative to improving or modifying the reduction technique itself is first to prepare even a relatively impure metal by a commercially viable reduction method and follow it by a suitable refining procedure. This is important because fairly goodgrade rare-earth metals with purity adequate for many application requirements are usually available at reasonable prices, and the metal that qualifies as pure for one application could be impure for another application. This also highlights the need for customizing the refining procedure for a particular application, with commercial metals as the source material. The refining step is, therefore, a necessary and important operation in the extractive flow sheet of the rare-earth metals.
